Chondrichthyan scales are rare in Lower-Middle Palaeozoic rocks.
Chondrichthyan-like scales, however, were described by Sansom et al.
(1996) from the Ordovician of Colorado, USA, and by Young (1997) from
central Australia. The earliest Elasmobranchii gen. nov. has been listed
from the Early Llandovery (Early Silurian) of the Siberian Platform,
Russia (Karatajute-Talimaa & Predtechensk)j 1995). Niualepis and
Elegestolepis conica have been identified in the Middle Llandovery of
South Yakutia and the Bratsk Region, Siberian Platform (Novitskaya &
Karatajute-Talimaa 1986; Karatajute-Talimaa 1992). Elegestolepis sp.
comes from the Upper Llandovery of Irkutsk Amphitheatre, Tuva, and North
Mongolia (Vladimirskaya et al. 1986).

Scales of an elasmobranch (Kannathalepis) and possible
chondrichthyans (Frigordepis and Wellingtonella) originate from the
Wenlock of the Canadian Arctic (Marss & Gagnier 2001; Marss et al.
2002, 2006). Elegestolepis grossi has been recognized in the Upper
Silurian (Upper Ludlow-Pridoli) of Tuva (Karatajute-Talimaa 1973). Four
chondrichthyan elasmobranch(?) genera, Seretolepis, Altholepis,
Knerialepis, and Ivanelepis, established on the basis of scales, have
been described from the Lochkovian of Podolia (Obruchev &
Karatajute-Talimaa 1967; Karatajute-Talimaa 1968, 1977, 1997a; Hanke
& Karatajute-Talimaa 2002). Polymerolepis Karatajute-Talimaa was
first discovered in the Lochkovian of Podolia (Obruchev &
Karatajute-Talimaa 1967). Later on it has also been identified in the
coeval strata of Central Nevada (Turner & Murphy 1988) and on
Ellesmere and Prince of Wales islands of the Canadian Arctic
(Langenstrassen & Schultze 1996). The list of chondrichthyan and
putative chondrichthyan taxa from the Lochkovian of the Mackenzie
Mountains (MOTH locality), Canada, includes Kathemacanthus, Altholepis,
Seretolepis, Polymerolepis species, and five unnamed taxa (Hanke &
Wilson 1997, 1998; Wilson & Hanke 1998; Wilson et al. 2000).
Ellesmereia are found in the Lochkovian of the Canadian Arctic (Vieth
1980). Scales of Arauzia, Lunalepis, and Iberolepis are known from the
Lower Devonian, Lower Gedinnian (Lochkovian) of Spain (Mader 1986).
Pamyrolepis nom. nud. Karatajute-Talimaa scales have been briefly
described from the Emsian of the Eastern Pamyrs (Karatajute-Talimaa
1992; Leleshus et al. 2005).

Mongolepidids were first treated as belonging to Elasmobranchii,
Chondrichthyes (e.g. Karatajute-Talimaa et al. 1990). Later on,
Karatajute-Talimaa (1995) suggested that Mongolepidida is an independent
group of lower vertebrates, still related to the Chondrichthyes. The
group is rather widely known in the Silurian. Mongolepis, Sodolepis, and
Teslepis have been described in the Upper Llandovery of West Mongolia
(Karatajute-Talimaa et al. 1990; Karatajute-Talimaa 1992;
Karatajute-Talimaa & Novitskaya 1992, 1997). Several taxa belonging
to the mongolepidids come from the Upper Llandovery of different sites
of China: Xinjiangichthys from Kalpin and Bachu counties, Xinjiang,
Shiqianolepis and two taxa of uncertain affinity, Rongolepis and
Chenolepis, from Shiqian county, Guizhou Province (Wang et al. 1998;
Sansom et al. 2000). In the Wenlock (Lower Silurian) of the Siberian
Platform Mongolepididae gen. nov. B has been identified together with
Elegestolepis? sp. (Karatajute-Talimaa & Predtechenskyj 1995). The
latest mongolepidids of the world were discovered in the probable
Lochkovian (Lower Devonian) strata of Eastern Mongolia (Wrona &
Nyamsuren 1998) and in the Lower Emsian (Lower Devonian) of Gornyi Altai
of Western Siberia, showing the wide stratigraphical range of the whole
group (Rodina 2002).

Scales of chondrichthyans are rare in the East Baltic. Lugalepis
multispinata comes from the Emsian(?) of Latvia (Karatajute-Talimaa
1997b), while the Eifelian (Middle Devonian) Narva Regional Stage (for
short, RS) of Leningrad District, Russia, Belarus, and Lithuania
contains a somewhat different form Lugalepis cf. multispinata
(Karatajute-Talimaa 1992, 1997b). Rare chondrichthyan buccopharyngeal
scales (= oral denticles) and teeth have been documented from the
Givetian (Middle Devonian) of Estonia and from the Famennian (Upper
Devonian) of Latvia (Ivanov & Luksevics 1994; Mark-Kurik &
Karatajute-Talimaa 2004).

In the course of lithological studies of Devonian sediments in
Estonia, based upon outcrops and drill core material, a rich collection
of vertebrate microfossils has been obtained. Notably abundant material
has been collected from the Karksi outcrop in the Halliste old valley,
near Karksi castle (Viljandi county) (Fig. 1). The sandstones here
belong to the lower part of the Harma Beds of the Burtnieki RS and are
famous for fish fossils. Vertebrates described in earlier studies
include psammosteids (Obruchev & Mark-Kurik 1965; Mark-Kurik 1968,
1995), placoderms (Karatajute-Talimaa 1963), acanthodians (Valiukevicius
1998), and sarcopterygians (Vorobyeva 1977). Chondrichthyans have not
been mentioned or described so far from these beds.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

In this paper we describe scales of a new chondrichthyan from the
Karksi outcrop, Estonia. The authorship of the new taxon Karksilepis
parva is attributed herein to Marss.

MATERIAL AND METHODS

Six samples (200-400 g) were collected from four levels of the
Karksi outcrop (Fig. 2) and examined. All samples, those previously
collected by A. Kleesment in 1990 and 1997 and the ones from two new
levels (samples 02-1 and 02-2) contained abundant vertebrate
microremains. Scales and tesserae or fragments of plates of
heterostracans, placoderms, acanthodians, chondrichthyans, and
osteichthyans (sarcopterygians and actinopterygians) were found (see
Table 1). Psammosteids predominated, especially plate fragments and
scales of Tartuosteus maximus, Pycnosteus tuberculatus, and Ganosteus
stellatus. The diversity of acanthodians was high (16 taxa), and
Nostolepis gaujensis scales were found for the first time in the
Burtnieki RS (they were absent only in sample 02-3). Chondrichthyans
were fairly rare but present in all six samples. In addition to the
scales of the new taxon described herein, the samples contained teeth of
an elasmobranch. As there was no indication that these teeth might
belong to the taxon described below (except their co-occurrence in the
samples from the same outcrop), they will be described elsewhere.
Scales, plate fragments, and teeth of Sarcopterygii were found as well.
Actinopterygians were represented by rare scales of Cheirolepis sp.
(Kleesment et al. 2003; Marss et al. 2003).

The sandstones in the Karksi outcrop were very weakly cemented and
did not need acid preparation. Samples were carefully washed to remove
muddy (<0.01 mm) particles. Then the material was fractioned and
sieved; the size ranges were 0.1-0.25 mm, 0.25-0.5 mm, 0.5-1 mm, 1-2 mm,
and >2 mm. The two finest fractions were separated to heavy and light
parts using bromoform (2.89 g/[cm.sup.3]). The two heavy fractions
obtained, and the two coarsest fractions were investigated using the
binocular optical microscope. All microremains with well-preserved
sculpture and shape were picked out. The microstructure of
chondrichthyan scales was examined in thin sections or by submerging
elements into immersion liquids. A selection of scales was photographed
using the scanning electron microscope JEOL-JSM-940-A. The described
collection is housed in the Institute of Geology at Tallinn University
of Technology under the collection numbers GIT 383 + specimen number.

GEOLOGICAL BACKGROUND

The Middle Devonian Burtnieki RS is distributed in southeastern
Estonia (Fig. 1). The total thickness of the stage is 60-95 m. The
sequence is represented by light (white, yellowish, pinkish, and
greyish-brown) fine-grained weakly cemented cross-bedded sandstones with
siltstone and clay interlayers. The stage is divided into three
successive cyclic sandstone complexes, the Harma, Koorkula, and Abava
beds (Kleesment 1995; Kleesment & Mark-Kurik 1997). The lower unit,
the Harma Beds, is 13-28 m thick. The beds cropping out in Karksi belong
to the lower part of the Harma Beds. The cross-bedded sandstones of the
Harma Beds with the stratotype section in Harma settlement at the Ohne
River are usually well sorted and fine-grained, where the size fraction
0.1-0.25 mm clearly dominates (75-93%; Kleesment 1995). The sandstones
in the Karksi outcrop and coeval sediments at the Ahja River and in the
Torva area have the same composition as the Harma Beds. Sandstones of
the Harma Beds possibly accumulated in regressive conditions of the
Devonian basin in a fluvially dominated subaqueous delta plain
environment, which was under influences of low-amplitude eustatic and
tectonic movements, repeatedly interrupted by short subaerial periods
(Kleesment 1997; Plink-Bjorklund & Bjorklund 1999).

The part of the Karksi outcrop that yielded fish microremains is a
small sandstone scarp 3.5 m high and 6 m long. About 20 m upstream, in
the upper part of the slope there is another, smaller fish-bearing
locality. The levels with abundant fish microremains are represented by
moderately sorted fine- to medium-grained sandstones which contain both
particle types in equal amounts and up to 10% coarse and very coarse
detrital material. In the coarse-grained (diameter 0.5-2 mm) detrital
material, heterostracan, placoderm, and sarcopterygian plate fragments
are fairly worn. Acanthodian and chondrichthyan scales are concentrated
in beds with the grain size of 0.2-0.5 mm and are not worn, which gives
evidence of more rapid accumulation of sediment. Probably the reworking
and weathering of fish bones and plates took place in shore face
environments, where under low-amplitude eustatic movements, sea level
fluctuations caused repeated alternation of marine and non-marine
conditions and intense redeposition and reworking of the detrital
component. Interlayers with rich, well-preserved fish microremains
possibly deposited in the nearshore realm during short periods of
intense influx of detrital material. Such rapid burial in nearshore
deltaic conditions resulted in the accumulation of well-preserved fish
microremains. In these layers, besides moderate sorting of detrital
material among fine-grained particles, subrounded to subangular quartz
grains dominate (85-90%); in the coarse-grained fraction, rounded grains
make up only 25-30%, referring to intense influx and rapid deposition.
Such high concentrations of fish remains have been reported from
nearshore marine (Davies et al. 2007), deltaic (Mancuso 2003), and lake
deposits (Smith & Swart 2002).

Morphology. The scales are small to medium-sized, quadrangular,
with the shape varying from high rectangle to relatively low rhomboid
(Fig. 3A, B, E-K, N, P, Q). The length of the scales is 0.25-0.8 mm and
width 0.25-0.7 mm. The height of the basal plate is about half the
height of the scale (Fig. 3L). The scales are covered with odontodes
forming polyodontodia. The number of odontodes varies from three (Fig.
3Q) to eighteen (Fig. 3J); their width is 0.05-0.1 mm and length
0.15-0.4 mm. The odontodes are attached to the basal plate of the scale.
The rectangular high scales carry odontodes in one to two rows (Fig. 3A,
B, E, G); the odontodes on rhomboidal scales are less regularly arranged
in rows but still up to four rows can be counted (Fig. 3H, J, N),
whereas more anterior odontodes partly cover the posterior ones. There
is always lateral separation between adjacent odontodes (e.g. Fig. 3A,
E, K). The odontodes can be directed horizontally (Fig. 3M) or rise to
the posterior (Fig. 3K; see also Fig. 4). All odontodes are covered with
fine ultrasculpture of longitudinal striation which starts at the
conjunction of the odontode and basal plate and is directed upwards and
posteriorwards, towards the peak of the odontode (Fig. 3A-E, G, I, K,
M). The basal plate is larger than the area with odontodes. In some
scales the basal plate strongly protrudes anteriorly (Fig. 3A).
Relatively large deep pores open in the basal plate (Fig. 3G, K-M, P, Q)
and make the surface of the plate uneven. The shape of the basal plate
depends on the age of the scale. Vascular network canals open anteriorly
of the odontodes in high rectangular scales (Fig. 313, E, G) or around
the odontodes in rhomboidal scales (Fig. 3K-M, P, Q). The lower surface
of the basal plate can be either concave, smooth (Fig. 3F), or convex
(Fig. 3L, M, O).

A few rhomboidal to roundish scales have their basal plates covered
with several (two to seven) odontodes with a flat and smooth surface.
The length of these scales is 0.45-0.75 mm and width 0.3-0.7 mm (Figs
3S-U; 5D). The width of the odontodes is 0.1-0.2 mm and length 0.2-0.4
mm. The odontodes are oval (Fig. 3T, U) to oblong (Fig. 3S), more than
twice as long as their width. The odontodes lie close to each other
(Fig. 3T, U), some space may be between the anterior odontodes and the
neighbouring ones (Fig. 3S). Anterior odontodes partly cover the
odontodes situated behind them; their posterior apices are usually
broken (Fig. 3S). On the sides of odontodes the surface is finely
striated. The basal plate is convex, slightly larger than the crown
(Fig. 3S-U). The pores occur in the facets of the basal plate and on the
plate around the odontodes, making the surface of the plate uneven.

Karksilepis gen. nov. is similar to osteostracans by having bone
cell cavities in its exoskeleton. Osteostracans have distinct radial
structure of peripheral sections of the base of tesserae, which are
absent on the basal plate of Karksilepis gen. nov. In Karksilepis gen.
nov. the upper layer is comprised of orthodentine, in osteostracans (if
present) the equivalent layer consists of mesodentine. Karksilepis gen.
nov. differs from osteostracans in having a specific vascular canal
network in the basal plate below the odontodes. The ultrasculpture of
striation is present in thelodonts, heterostracans, osteostracans, and
in Karksilepis parva gen. et sp. nov. The convex basal plate of the
scales of Karksilepis gen. nov. is superficially similar to the scale
base of acanthodians but their crown sculpture and growth of the scale
are different.

The greatest similarities can be found between Karksilepis gen.
nov. and some chondrichthyan taxa. Chondrichthyans constitute a very
diverse group, with the representatives having very different features.
In the elasmobranch chondrichthyan taxa Elegestolepis
Karatajute-Talimaa, Kannathalepis Marss & Gagnier, and Ellesmereia
Vieth monodontodia are simple, the crown has one pulp cavity, one neck,
and one basal canal, while Karksilepis gen. nov. Marss has complex
polyodontodia. A possible elasmobranch Seretolepis Karatajute-Talimaa
has peculiar lamelliform odontodes on the crown, the base built from
aspidine, and basal and neck openings. Knerialepis Hanke &
Karatajute-Talimaa has the scales whose odontodes consist of mesodentine
and the basal plate of aspidine. Vascular canals are developed between
the basal plate and the crown, making Knerialepis similar to
Karksilepis, but the sculpture in the former consists of posteriorly
fusing wedge-shaped flat odontodes. In Altholepis Karatajute-Talimaa the
crown is formed of a great number of narrow anchor-shaped odontodes
pointed posteriorly; the scale crown is built from orthodentine and the
base--from aspidine. Ivanelepis Karatajute-Talimaa has scales with a few
oblong odontodes and neck and basal openings. Arauzia Mader is defined
as a possible hybodontiform taxon with rather high scales with upwards
directed odontodes of cyclomorial growth and vascular canal openings in
the neck and base (Mader 1986). The cladoselachid taxa Iberolepis Mader
and Lunalepis Mader have the scale crown and microstructure somewhat
resembling those of acanthodians (ibid.). The possible chondrichthyans
Frigorilepis Marss et al. and Wellingtonella Marss et al. have
monodontode scales without neck canals. In chondrichthyan scales, the
neck canals can be present or absent in the same fish, for example, the
fin scales of Polymerolepis Karatajute-Talimaa lack neck canals (G.
Hanke, pers. comm. 2008) while the scales of other parts of the body
have them (see also Karatajute-Talimaa 1998).

Karatajute-Talimaa (1997b) erected the elasmobranch order
Lugalepidida based on the characteristics of pectoral and pelvic fins,
complex scales in which the odontodes (polyodontodia) of the crown are
formed of orthodentine, and the non-growing base of acellular bone
tissue; there is no horizontal canal system on the boundary between the
crown and the basal plate. According to Karatajute-Talimaa (ibid.), this
order differs from Mongolepidida, whose representatives have scales with
a growing base and odontodes of lamelline forming longitudinal
odontocomplexes of the crown. Karksilepis parva gen. et sp. nov. is
externally rather similar to Lugalepis multispinata, which comes from
the Emsian(?) and Eifelian of Lithuania, Latvia, Belarus, and
Kaliningrad District, Russia (Karatajute-Talimaa 1997b). Both taxa have
scales whose crowns consist of numerous spiniform odontodes.
Karatajute-Talimaa (1997b) defined such scales in L. multispinata as
polyodontodia of synchronomorial type. Nevertheless, these taxa differ
in that the upper surface of odontodes of L. multispinata has a
longitudinal groove in the medial part, while K. parva gen. et sp. nov.
possesses spines with round cross section. Only K. parva gen. et sp.
nov. has ultrasculpture of fine striation. The basal plate of the scales
of L. multispinata consists of a spongy acellular bone tissue and thin
basal lamina, but our new taxon has flattened bone cell lacunae forming
the lamellar structure of basal plates.

The scales of Cladodontida also are formed of orthodentine and the
basal plate of scales is either of cellular bone tissue (in Cladolepis)
or acellular (in Ohiolepis and Protacrodus) (Gross 1973).
Karatajute-Talimaa (1997b) noted that some Mongolepidida (Mongolepis,
Sodolepis) have an acellular bone developed in the base of scales and
some (Teslepis) have a cellular bone with fusiform bone cells; the
horizontal system of canals is present in both Mongolepidida and
Cladodontida. It would be most appropriate to accommodate Karksilepis
gen. nov., having orthodentine in the complex odontodes, horizontal
canal network beneath the odontodes in the basal plate, and a cellular
bone in the base, within the Cladodontida. Difficulties rise when
considering the growth of the crown. In Cladodontida odontodes of
approximately the same size were added areally, forming rather regular
ring-like zones (Karatajute-Talimaa 1992), while in Karksilepis gen.
nov. they occur in irregular rows. As a conclusion, at present we prefer
to leave the family and order for Karksilepis parva gen. et sp. nov.
Marss open.

RESULTS

Based on the features of scale sculpture, the vascular canal
network, and the placement of the canal openings beneath the odontodes
in the basal plate, a new chondrichthyan of uncertain order and family,
Karksilepis parva gen. et sp. nov. Marss, is established with specimens
from the Harma Beds, Burtnieki Regional Stage, Karksi outcrop, Estonia.
This is the first time that chondrichthyan dermal scales were discovered
in Estonia. Previously only chondrichthyan buccopharyngeal denticles
have been documented from the region (Mark-Kurik &
Karatajute-Talimaa 2004). The teeth of an elasmobranch from the same
outcrop will be discussed in another study.

ACKNOWLEDGEMENTS

We thank J. Valiukevicius for identifying acanthodian scales from
Karksi and V. Talimaa (both from Vilnius) for the discussions on the
early chondrichthyan scales, and A.Ivanov, St Petersburg, for reading
the early version of the manuscript and useful suggestions. The referees
Drs Ivan J. Sansom, Birmingham, UK, and Gavin F. Hanke, Victoria,
Canada, are thanked for their advice and comments on the original
manuscript. SEM images were taken in the Centre of Materials Technology
at Tallinn University of Technology (TUT) by Dr. V. Mikli. Thin sections
were digitally imaged and all figures improved for the publication by G.
Baranov, Institute of Geology at TUT. We thank both for their help. The
study was supported financially by the Estonian Science Foundation
(grants Nos 5726 and 7334) and by Estonian Target Funding projects
SF0140020s08 (for TM) and SF032008OsO7 (for AK).

Kleesment, A., Marss, T. & Niit, M. 2003. The Devonian of
Estonia and the study of its vertebrate microfossils at the beginning of
the new century. In The Fourth World Meeting of Estonian Geologists.
Estonian Geology in the Beginning of the New Century. Conference
Materials and Excursion Guide (Plado, J. & Puura, L, eds), pp.
31-33. The Geological Society of Estonia and Institute of Geology,
Tallinn, and University of Tartu, Tartu [in Estonian].